Spot welding is a known welding technology for welding together metal plates, which can be used in the manufacture of automobiles, domestic electrical products, and the like. In spot welding, firstly, as illustrated in FIG. 18, two superimposed metal plates 100a, 100b are sandwiched between a pair of electrodes 150a, 150b. In this state, pressure is applied locally to the metal plates 100a, 100b by means of the pair of electrodes 150a, 150b, and current is passed between the electrodes 150a, 150b. The current flows in a concentrated manner through the portion of the metal plates 100a, 100b sandwiched between the electrodes 150a, 150b, and therefore generates Joule heat. A portion of the metal plates 100a, 100b is melted by this Joule heat, whereupon, the passage of current is halted. When the molten portion of the metal plates 100a, 100b cools and solidifies, the metal plates 100a, 100b will be welded together.
FIG. 19 is a cross-sectional view of a spot welded section of two metal plates 100a, 100b which have been spot welded as described above. In the spot welded section, the outer surfaces of the metal plates 100a, 100b are dented due to the pressure applied by the electrodes 150a, 150b. This denting is called an “indentation” 101, and the length L1 thereof is called the “indentation diameter”. A nugget section 102 and a pressure bonded section 103 are formed in the spot weld section. The nugget section 102 in a region where the metal plates 100a, 100b have become unified as a result of being melted due to the application of heat and pressure, and then solidifying. The length L2 of the nugget section 102 is called the “nugget diameter”. This nugget diameter L2 greatly influences the welding strength achieved in the spot weld section. The greater the nugget diameter L2, the greater the weld strength of the spot weld section. The pressure bonded section 103 is a region which has received the effects of the applied heat and applied pressure and where the metal plates 100a, 100b have bonded together under pressure. The total length L3 of the nugget section 102 and the pressure bonded section 103 is called the joint diameter. The pressure bonded section 103 is surrounded by a thermally annealed heat affected zone (HAZ) 104. The HAZ 104 has a length L4 called the HAZ diameter. The HAZ 104 is surrounded by an original metal 105 whose metallographic structure has not been affected by the spot welding.
Generally, the nugget diameter L2 or the joint diameter L3 in the spot weld section achieved by welding is appropriately 10 millimeters or less, which is relatively small. Therefore, in many cases, it is necessary to inspect the spot weld section in order to check that it has sufficient weld strength. Since the weld strength of the spot weld section is greatly influenced by the nugget diameter L2, then the nugget diameter L2 can be used effectively as a basis for judging whether or not the spot weld section has a suitable welded state.
Japanese Patent Application Laid-open No. Hei10-26609 discloses inspection technology, one object of which is to measure the nugget diameter L2 in a non-destructive manner, and to judge the suitability or unsuitability of the welded state of a spot weld section on the basis of these measurement results. According to this patent publication, an excitation coil is disposed in the vicinity of an inspection target, and a loop coil forming a sensor is disposed between the inspection target and the excitation coil. In this state, a static magnetic field is generated which passes through the inspection target and the sensor, by passing a DC current through the excitation coil. Thereupon, when the static magnetic field is shut off, the inductance of the loop coil (or a physical quantity that is directly proportional to the inductance thereof) is determined by tracing the course of the loss of the electrical field remaining in the inspection target. This inductance indicates the magnetic permeability of the nugget section 102 and pressure bonded section 103, or the like, constituting the spot welding section through which the residual magnetic field passes. When measurement of this kind is carried out in a plurality of positions with respect to the inspection target, then variation will occur in the plurality of inductances obtained. This variation in inductance reflects variations in the internal structure of the spot weld section. Therefore, the nugget diameter L2 can be estimated by detecting the variations in magnetic permeability, and hence the variations in inductance, caused by changes in the internal structure of the spot weld section, by means of non-destructive inspection technology.
According to the Japanese Patent Application Laid-Open No. 10-26209, the sensor loop coils are arranged to face the indentation 101 at a predetermined distance from the metal sheet 100a or the metal sheet 100b in the non-destructive inspection. Under normal circumstances, air is present at the indentations 101. The magnetic permeability of the air is much smaller than the magnetic permeability of the nugget section 102 or the pressure bonded section 103. Accordingly, the measured loop coil inductance reflects not only the magnetic permeability of the internal structure of the spot weld section but also the magnetic permeability of the air at the indentations 101. If the inductance (or a physical quantity proportional to the inductance) reflects the magnetic permeability of elements other than the internal structure of the spot weld section, the non-destructive inspection of the spot weld section may not give an accurate estimation on the nugget diameter L2.